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The Design Flaw That Almost Wiped Out an NYC Skyscraper (slate.com)
232 points by x43b on Apr 17, 2014 | hide | past | web | favorite | 88 comments

What this story leaves out is that the EVP of Citicorp at the time was a MIT-trained scientist (physical metallurgy). So when this crisis bubbled up - there was no hesitation in action since there was a scientist/engineer in top leadership who was able to communicate to the board the severity of the situation.

"Together they flew to New York City to confront the executive officers of Citicorp with the dilemma. "I have a real problem for you, sir," LeMessurier said to Citicorp's executive vice-president, John S. Reed. The two men outlined the design flaw and described their proposed solution: to systematically reinforce all 200+ bolted joints by welding two-inch-thick steel plates over them."



So I presume the bolts and added welded plates were actually stronger than the originally specified welded joins?

In the interview with Bill LeMessurier it seemed the problem existed even with the original joins - the bolted joins just made the issue significantly worse...

Yes, it is more akin to bridge construction.


"The building now stands as one of the safest skyscrapers in New York City, able to withstand a 700-year[1] storm without the aid of the tuned mass damper."


[1] Approximately 140mph gusts. http://civil.unm.edu/classes/content//CE%20491-002/ASCE%207-...

This must have been in the golden age before Citigroup was run by idiots.

One point that this article doesn't mention but the video does (starting about here: http://www.youtube.com/watch?v=TZhgTewKhTQ#t=350) is that the building wasn't built exactly as designed.

In particular, the 8-story-high diagonal parts were done in multiple splices that were supposed to be welded together but ended up being bolted together. It sounds like it made things much worse.

This reminds me of the 1981 Hyatt Regency walkway collapse [1]. The blueprints called for continuous suspension rods with walkways attached along their length. This would mean that the load on any single joint between rod and walkway would be limited to the load on one walkway. But as built, the rods were not continuous, rather they connected each walkway to the one below. This meant the load on the topmost rod-walkway joint was the aggregate of the load of all walkways hanging below it.

The builders did not notify the designers of the change, and 114 people were killed as a result.

[1] http://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse

And not only that, instead of using a box section for the steel piece, they used two c sections welded together like this: []. A box section is strong, that built up section, not so much.

The thing is, that design would have been a horrible pain to build as designed. It's a design failure. To build it, you'd need to slide a large steel section up two screws, following it by a nut up 30 feet. It needed to be redesigned by someone who knew more about fabrication and erection, and then checked closely to make sure that there weren't material changes in the performance.

> To build it, you'd need to slide a large steel section up two screws, following it by a nut up 30 feet.

This is fascinating, but I feel that I'm missing some terminology and concepts. I wonder if you could explain in more detail and clarify the terms?

From the part I do understand, it reminds me a lot of what I've encountered in a recent software project or two.

A company hires a visual design consulting firm for hundreds of thousands of dollars, and boy do they get their money's worth. Beautiful images and designs, complete with high quality videos with things moving all over the place in the smoothest and most seamless way.

And not a thought toward how this fantastic beautiful design would actually be implemented. No consultation with the programmers to see what could actually work given the required technology.

Agile? That's for programmers. When it comes to the product design, the visual designers have spoke, and that is that. It's waterfall time, baby!

On one project the designers decided it would be beautiful to have menus and controls that would slide out and overlap a Google Earth plugin. Great idea! Until you realize that it would take three solid months to work out all the cross-platform bugs in that approach. Three months that could have gone into building something useful, something that customers actually cared about.

Yes, physical construction and software construction can have some of the same communication difficulties between designers and builders.

When I worked as a CAD operator for a company which fabricated glass doors and windows, I would often receive printed drawings from architects. Soft copies were not available, as the architects considered their designs to be proprietary. But of course we the fabricators would benefit from having the design in CAD so we could produce different views and so on.

One day I received a set of drawings for a three-dimensional arrangement of glass sort of like a bay window. There were plan (overhead) and elevation (side) views, and I stared at those for a while, unable to make a coherent 3D model to match them. I then took some cardboard and cut it out in the shapes shown on the drawings. The shapes did not actually fit together--any way you tilted the pieces, there would be unworkable gaps in some part.

This was at the time when a lot of drawings were still made in 2D, with manual work to align the different views. I ended up having to visit the other firm's office, my cardboard cutouts in hand, to show them that what they had drawn could never be built.

This is fascinating, but I feel that I'm missing some terminology and concepts. I wonder if you could explain in more detail and clarify the terms?

So, the atrium was (say) 80 feet tall, with the sky bridges every 20 feet. So one at 20, 40, and 60 feet. If the continuous rod that had been specified in design was used, it'd be a little longer than 60 feet long (80 foot ceiling, lowest bridge 60 feet below that, plus a another foot or so to make room for fasteners).

That would mean that the middle bridge would have to have had the nuts spun along 40 feet (either from the top or bottom) and the nuts for the topmost and bottom-most bridge would have to be spun along 20 feet of thread. But before you could put the topmost nut on, you'd have to support the rod as you placed it through the box-section beams for the middle bridge. And then do the same for the topmost beam. And then lift it all so that the top end of the rod could be secured to the ceiling.

And this wouldn't have been one rod at a time -- you'd have to do the same for all dozen or more rods at the same time. Nightmare from a construction schedule standpoint.

My understanding of the incident also is that 60+ foot rods didn't, and still don't, exist as regular items. They would have needed to be custom-made, further hitting cost and schedule.

Could something like this be used (at a large scale) to "extend" multiple shorter segments of rod? http://www.pl-259.com/nuts%20adapt%203%208%20to%201%204%20x%...

Yes. Couplers would work(1). You'd probably want jam nuts as well, just so things don't back out on you.

1. There may be problems with that approach. IANAPE. They would be different problems than actually caused the failure.

I had one like that in 2000 when I was implementing web designs. I was given an elaborate design requiring javascript mouseovers and fancy doodahs. My "it's too elaborate" was brushed aside and even with my best efforts the first page visit took 2 1/2 minutes to grab the assests.

I did a version in plain text and they ended up offering me a job.

Hentry Petroski's To Engineer Is Human: The Role of Failure in Successful Design [1] has a chapter on the Hyatt Regency. The whole book is great.

[1] http://www.amazon.com/To-Engineer-Is-Human-Successful/dp/067...

I work in a prototyping and fabrication shop, and this is such a common problem. Engineers need shop floor/worksite experience to understand problems at the level the workers see them, but so few have that experience. We regularly receive designs that are impossible or impractical to build. Engineering schools don't seem to have time to teach students this stuff, so while they are "smart" and understand the math, they fall flat when it comes to the "simple" stuff.

I had a "Principles and Practices" class in my undergrad (EE) that used the Hyatt Regency walkway incident as a case study for this exact reason. Raytheon (my first employer) also had a habit of putting EE and ME new grads on production support, so that they could become familiar with the company's products as well as be more conscious of testability and manufacturability issues with designs.

Ah, it's good to hear that. That sort of experience and awareness is absolutely crucial.

My father is engineer, once he was renovating a shop in a intersection, he told the builders to use five tree trunks to hold the second floor while they demolished and rebuilt the first floor merchandise display. The builders decided to use only three, and pocket the money of the other two to themselves, and the building fell, thankfully my father had started working before signing the contract, so he could not be sued.

Can you sue the engineer if the builders don't follow the spec?

On Brazillian law the engineer is held responsible for everything that can go wrong, unless he proves it was not his fault...

This mean in Brazil there is a lack of experienced and honest engineers (the scumbag ones know how to not get shafted, the honest ones go to other countries, and the non-experienced ones... well, more meat for the grinder).


Earlier in May, LeMessurier met for an inquiry on another job where he mentioned the use of welded joints in the Citicorp building, only to find a potentially fatal flaw in the building's construction: the original design's welded joints were changed to bolted joints during construction, which were too weak to withstand 70-mile-per-hour (113 km/h) quartering winds. While LeMessurier's original design and load calculations for the special, uniquely designed "chevron" load braces used to support the building were based on welded joints, a labor- and cost-saving change altered the joints to bolted construction after the building's plans were approved. Base of the Citigroup Center View from the street

The engineers did not recalculate what the construction change would do to the wind forces acting on two surfaces of the building's curtain wall at the same time; if hurricane-speed winds hit the building at a 45-degree angle, there was the potential for failure due to the bolts shearing. The wind speeds needed to topple the models of Citigroup Center in a wind-tunnel test were predicted to occur in New York City every 55 years. If the building's tuned mass damper went offline, the necessary wind speeds were predicted to occur every 16 years.

The fix was to rip the building apart and weld everything together.

According to wallflower below, it was to weld thick steel plates to the joints. That's not the same as following the original design.

Reminded me of the design flaw of the Millennium Bridge in London, where the engineers accounted for all resonance modes but one, the one that can be caused by pedestrians:

> Resonant vibrational modes due to vertical loads (such as trains, traffic, pedestrians) and wind loads are well understood in bridge design. In the case of the Millennium Bridge, because the lateral motion caused the pedestrians loading the bridge to directly participate with the bridge, the vibrational modes had not been anticipated by the designers. The crucial point is that when the bridge lurches to one side, the pedestrians must adjust to keep from falling over, and they all do this at exactly the same time.


Accounting for "all resonance modes" is physically impossible as there are an infinitely many number of resonance modes.

It's more precise to say "accounted for most dominant resonance modes".

"Accounting for" means they made sure it was strong enough for the expected loads.

There was a similar issue with pedestrian ramps at Lincon Financial Field in Philadelphia, where the Eagles play.


"But what I found out at that meeting were that all factors of safety were gone."

Many catastrophic "accidents", and I use quotes because they could have been averted had people not cut corners w/o knowing the full context.

  * Chernobyl (after hours test by an untrained crew with an inverted fail safe design)
  * http://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse (design change in the field, very similar to the citicorp flaw)
  * 3 Mile Island (indicator that triggered on switch rather than valve)
  * Fukashima (cost cutting on seawall and generator snorkels)
  * Ariane-5 (code reuse, dead code)
If you want to look at good engineering, look at the Brooklyn Bridge[5] and the DC-3[6].

Too many people don't design with proper safety factors. You build it, you test it, you test it till it fails and you understand those failures. I would trust another citicorp wouldn't happen because we can do realistic wind model, we can do an earthquake model, an anything model. Maybe we can get to a safety factor of 1 when everything is automated, when everyone has an off-site backup of their own brain but until then. Safety factor 6.

[5] http://www.asce.org/People-and-Projects/Projects/Landmarks/B... [6] http://en.wikipedia.org/wiki/Douglas_DC-3

The Brooklyn bridge is over-engineered. Proof of that is that it handle today's loads.

Of course, the advantage of that is that it hasn't collapsed yet, like a few under-engineered bridges built way later (http://www.washingtonpost.com/wp-dyn/content/article/2007/08...)

But I doubt the ones paying for the Brooklyn Bridge would have chosen this design if they could have had something for half the price that would last for 75 years.

As a European this argument is a bit strange to me (since we have so many structures that are hundreds of years old and are still in use today). Don't people sometimes just want something that works and keeps on working? A monument of engineering to be proud of? Something to show to visitors? And on the economic side of things I'm not at all convinced that rebuilding every century would be cheaper for something like a bridge, considering the cost of closing it down.

Want? Yes, but want to pay for? I like it that I can see a 600+ year old church from my living room, too, and that affected the price of my apartment, but I don't think I would be willing to pay much to have a new building that will be considered iconic in 500 years time in my view.

Most modern bridges (and buildings, for that matter) get designed for 75, maybe 100 years of life. I don't think that is different in Europe. Older ones that still stand typically are sturdier, partly due to the use of larger safety margins by engineers who (according to today's knowledge) didn't know much about materials science, partly due to natural selection (bad designs collapsed or were taken down decades ago)

For every centuries old bridge, scores have been demolished because they couldn't handle increased traffic or just became too expensive to operate. And that happens in Europe, too, even in old-stuff loving Great Britain (http://en.wikipedia.org/wiki/London_Bridge)

Looking at bridges, I think the only ones that will stand for centuries are stone and masonry ones, and those rarely are built anymore. It is way easier to get large spans using reinforced concrete or steel. Both contain metal that rusts. Preventing that is expensive; fully preventing it probably prohibitively so. Even for landmarks bridges such as the Firth of Forth the Brits do not aim for eternal life (http://en.wikipedia.org/wiki/Forth_Bridge#Maintenance: "Network Rail has estimated the life of the bridge to be in excess of 100 years. However, this is dependant [sic] upon NR’s inspection and refurbishment works programme for the bridge being carried out year on year")

If masonry is able to withstand the weather an order of magnitude longer than steel - wouldn't it make sense to reinvent masonry building techniques for building stuff that's expensive to close down? I get that labor cost is prohibitive for this kind of stuff - maybe we will see a renaissance of masonry once we have flexible enough robots to handle it?

I'm not an expert on this, but I wouldn't know of "stuff that is expensive to close down" relative to the extra costs of a masonry bridge.

Let's consider the Golden Gate Bridge as an extreme example (extreme because lots of traffic across it has no reasonable alternative routes). If we needed to replace it, we could build a replacement bridge next to it, connect roads, and then open them over a weekend with little disruption.

If we chose to shove the new bridge in place of the old one in a week or so, that would have to disrupt traffic, but I doubt it would be more expensive than building _and_paying_for_ 20+ piers instead of the two we have _now_ (looking at http://en.wikipedia.org/wiki/List_of_longest_masonry_arch_br..., we cannot build masonry spans over 100m. The arch bridge is the best we can hope for when using stone because stone isn't strong in tension).

Also, such a disruption, announced years in advance, need not be that much of a disruption. People will take a few days of, stay with friends or family, maybe a temporary camp will be set up, etc)

On the other hand, that Wikipedia page mentions that many 50m+ stone arches have been built in China since 1950.

The Golden Gate Bridge is actually a lot less critical than you might imagine. For a really critical bridge, consider the neighbouring San Francisco/Oakland Bay Bridge, which carries about twice the daily traffic. And in fact, the eastern span of the Bay Bridge was replaced last year.

The reason many stone and masonry structures are still standing is two-fold.

One: most of them are not impressive from a structural point of view. Two: most of them have been retrofitted so many times that they're hardly the "original" structure.

Older bridge structures in the US are like this too -- especially old viaducts and masonry bridges built for trains.

They were overbuilt because they didn't have the tools to estimate and calculate loads well, and didn't have alot of choice in materials. Essentially the choices were wood trusses or masonry arches.

I agree with you about lifespan -- look at New York's Tappan Zee bridge as an example -- it's a major traffic corridor, with a bridge with a lousy 50 year lifespan. Replacement will cost something like $10 billion!

It's something deep in American culture I guess. Thomas Jefferson was worried 200 years ago about all the buildings we were making in wood. He predicted the nation would have no architectural heritage, a tabula rasa after 100 years when the wooden structures have to be replaced. Many of the buildings Jefferson designed and built back then then are still going strong, but I guess that sentiment never caught on here.

As an American, I'm extremely curious. What does your nationality have to do with that comment?

I'm a bit confused about your comment.

1. I didn't even mention my nationality.

2. Right in my post I explained, why growing up in Europe might change perspective, since we're surrounded by old structures still in use.

3. To extend on this point: I do believe that the thinking here is a bit different when it comes to longevity of buildings. Hence why most homes here are made out of bricks or concrete with heavy foundations and a cellar, while the typical American home seems to be built out of wood and doesn't have a cellar. The abundance of land is probably heavily factoring into this, so more people can afford to buy land and build a house on it[1].

[1] E.g. Switzerland, my home country, has a home ownership of only 34% vs. 67% in the USA.

The occurrence of cellars is dependent more on the geography in the US. For example in Florida where the water table is very high, it makes little sense to dig a cellar since it would constantly have to be pumped out. However in the midwest and north east cellars are quite common.

Your statement is depressingly true. I see this as trend where the builder is not the owner so costs are cut that an owner would never agree to. You see this in modern buildings all the time. It also might be that older structures like the Brooklyn Bridge are in-effect self insured. No one is going to get paid when it falls down, so it doesn't fall down.

The Bridge started carrying train traffic 6 years after it opened so I am not sure today's loads are any lighter.

Another direction would be to build by default with as much margins as realistic.

The original design with the welded joints would have allowed the building to withstand the corner winds, but it was replaced by bolts because builders thought it was overkill. Perhaps the consensus (or law?) should be to build with at least 2-3 times the calculated resistance, with only some justified exceptions.

Buildings would cost way more, but Intuitively it wouldn't be a bad thing (they would last longer and they would a bit less of a "mine's bigger" attitude)

Safety factors also exist because we don't know what we don't know. Models sometimes break down unexpectedly, so a factor >1 is sort of a "meta safety factor 1", taking into account that our models/people somewhat predictably fail.

The idea of a "factor of safety" has gone away in structural engineering to be replaced by a similar concept of "factoring" (look up allowable stress design, ASD, and load resistance factor design, LRFD, for more information). Factors are based on statistical analysis and criticality of failure.

No combination of factors approaches anything near what you would call a safety factor of 6. Typically things are designed with an analogous safety factor of 2.5 or less.

This isn't to say that higher factors of safety aren't possible, it's just that they're not worthwhile. We can design for anything. We just can't pay for it most of the time.

This is fascinating, http://www.onlineethics.org/cms/24528.aspx

It also reminds me of the Mars Climate Orbiter. http://en.wikipedia.org/wiki/Mars_Climate_Orbiter

Another good source is: http://www.damninteresting.com/a-potentially-disastrous-desi...

Apparently it costed $8 million to repair.

Joe Morgenstern's 1995 _New Yorker_ article "The Fifty-Nine Story Crisis" is also an excellent telling of this story.


Use asterisks for emphasis, like this: New Yorker

This fantastic validation for a person's undergraduate thesis. It's a real shame that she wasn't given proper credit at the time.

That's really scummy that the student did not receive appropriate credit at the time or after. That should have been a career maker story.

The 99% Invisible story covers that a bit if you're interested. http://99percentinvisible.org/episode/structural-integrity/

Reminds me of: http://en.wikipedia.org/wiki/File:CCTV_Beijing_April_2008.jp...

I'm sure the CCTV building is safe, but I get a small panic attack just thinking about walking or jumping up and down in that overhanging corner of the building.

How did they fix it? The article says they "welded" but doesn't say what was added to increase the strength of the building.

The documentary is interesting. Some of the steel structure held together by bolts were welded together with some extra steel braces.

This has always been a fascinating story - I can't believe Slate's just now discovering it. There's a much longer, more detailed account somewhere.

There's a pretty good one in the New Yorker (paywall, though I've seen scans of the article floating around): http://www.newyorker.com/archive/1995/05/29/1995_05_29_045_T...

A full text of the article can be found here:


This is much more comprehensive and interesting than the source linked in OP and elsewhere.

I discovered this a few years back while doing some reading for an ethics class, and was fascinated by it.

It was dramatized in the "Structural Corruption" episode of NUMB3RS, though they added in a suicide / murder element.

Yeah. I work in this building and it terrifies me everyday. It does sway in the wind. You can hear the walls creek when the winds are high. Besides the complete fear that is instilled in my heart because this building was built in a flawed way it also has an absolute terrible elevator design.

The elevators are double-deckers. So the elevator actually has two rooms that move together. In the morning if you want to go to an even floor you have to enter from the basement level and if you want to go to an odd floor you have to enter from the lobby level. This results in the weird sensation that if you are going to a higher floor the elevators often stop but the doors do not open. Probably related to the weird design the elevators break down often and with or without an elevator set out of commission there are high queue times during peak hours. This is a building that could REALLY benefit from an elevator call system where you enter a floor and it assigns you which elevator to get into to. Why they haven't installed a relatively simple fix like this is beyond me.

My final gripe about this building is its sheer ugliness IMHO. The steel facade pegs it as one of those buildings that was built in a particularly time. It's not timeless like the Empire State Building or perpetually modern like the Willis Tower or One World Trade center.

I guess its only redeeming factor is the very nice public plaza it has both inside and outside, its direct access to the E and 6 trains (super convenient for me), and the fact that it has not fallen down yet.

If only software flaws in large commercial proprietary/closed source systems were subject this kind of discovery and mitigation before things break.

The flaws in computer software cause damage because people are quick to exploit them. Mother nature is not quite as driven by money and corruption.

A terrifying thing is that one could hack a building management system or the controls for the tuned mass damper specifically and essentially "self destruct" the building, or at least make it much more vulnerable to mother nature.

I'm curious: since this flaw was pointed out in 1977, has NYC had such winds? How many times? Would Sandy have done it?

I think there was an episode of Numb3rs that was based on this. Maybe Season 1 episode 4 (Structural Corruption)?

It was fun to see the premise turn up in the show and go, oh, I think I know where they got this idea from.

Maybe this is covered in the New Yorker article marklabedz linked to, but I'm curious what their solution to quartering winds was. (They mention welding as a part of the solution but don't go into more detail.)

I found the answer in the Wikipedia article[1]:

> For the next three months, a construction crew welded two-inch-thick steel plates over each of the skyscraper's 200 bolted joints during the night, after each work day, almost unknown to the general public.

[1] http://en.wikipedia.org/wiki/Citigroup_Center

DRTA, but I listened to the podcast. In it, they say that if the Citicorp tower fell, it would topple nearby skyscrapers like dominoes.

Out of morbid curiosity, what's the worst case scenario for something like this? Let's assume terrorists could detonate a bomb large enough to knock over any one skyscraper in the world. Where should they put it to maximize the chain reaction of destruction?

I think most skyscrapers fall straight down under the load rather than topple. Similar to what happened to the WTC towers. Not that I have any credibility in this area, but according to that documentary people seemed especially concerned that this particular building would topple instead of collapse straight down.

I wish the author had referenced "Why Buildings Fall Down," the book that first made me aware of the Citicorp Building problem: http://www.amazon.com/Why-Buildings-Fall-Down-Structures/dp/...

Maybe this is a dumb question, but why couldn't they have found another location for the new church? Why did it have to be on the same plot as the Citicorp building?

It was an agreement between the church who owned the land and the Citicorp company that wanted to build a building on it.

Sure you could have move the church, but where would you put it? It's in downtown Manhattan. You could probably find space pretty far away, but it's unlikely church members would stay members.

It was an old church. The chuch allowed the Citicorp building to be built around it, which ended up being above it.

EDIT: spelling

They tore down the old church and built a new, modern one on the site. This is what makes this whole story so absurd to me.

Churches are parts of their community. Moving them isn't always an option. In NYC the Catholic churches are particularly tied to their neighborhoods, historically, so there really aren't many places to move to and still provide a place for that same community.

The church could have been established as floors 2 through 10 (or indeed, floors 70 through 80) of a conventional rectangular box skyscraper, instead of building what is effectively a large shack at ground level, and designing a risky skyscraper without corner supports because one of those corners would pierce the shack.


That makes logical sense in the same way as turning central park into a "park skyscraper" makes logical sense.


Where in the Bible do they demand masonry construction and a lack of elevators?

And when the poor and homeless show up looking for the soup kitchen and temporary shelter? That should fly real well with the monied gentry on the 50th floor.

A bug that would make even Heartbleed blush.

This sort of story never gets old.

This podcast is fantastical.

More interesting reading around "Tuned Mass Dampers" http://en.wikipedia.org/wiki/Tuned_mass_damper

The first I had heard of a tuned mass damper was in the installation on Taipei 101. There's a good video of it moving that gives you an idea of just how colossal these things are: https://www.youtube.com/watch?v=NYSgd1XSZXc

What I also find very interesting are the "tuned column liquid dampers", which applies the same idea of "tuning" the period of the liquid system to one period of the structure. They are found to be very effective in dissipating energy after seismic events or constant wind. I'm a civil engineering student (currently writing my thesis) and I just loved this systems when I learned from them.

The first Tuned Mass Damper was in the John Hancock Tower.


From an engineering standpoint that is a pretty depressing read.

That was a great read, I usually HATE stories that come from Slate but this partially redeemed them in my mind.

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